1. Field of the Invention
This invention generally relates to optical device integrated circuit (IC) fabrication and, more particularly, to a light emitting device and planar waveguide using a single-sided periodically stacked interface.
2. Description of the Related Art
Free space optical communications and optical interconnector applications require directional emissions from a light source in order to achieve high collecting efficiencies for good system power budget designs. Using Si nanoparticles as light emission centers inside silicon rich silicon oxides, SiOx (x<2) light emitting devices can be used in direct modulation modes in free space optical interconnector applications. However, when an active SiOx (x<2) layer is sandwiched between a transparent ITO (Indium Tin Oxide) top electrode and a p or n-doped silicon substrate as the other electrode, the emission efficiencies into air are poor. The poor efficiencies may be the result of two major mechanisms. First, the loss of most of emitted light into the highly doped Si substrates due to its high optical index. Second, the emitted light through the top ITO layer is not collimated, which leads to poor collection of emitted light by photodetectors. A photodetector with a fixed cross section can only cover a very small range of emission angles when the distance between the photo detector and SiOx emitter are very large, as compared to the size of SiOx emitters.
FIGS. 14A and 14B depict a silicon light emitting device and its photoluminescence and electroluminescence spectrum (prior art). A simple light emitting device is schematically shown in FIG. 14A with active Si nanoparticles embedded in a dielectric layer of SiOx, which is sandwiched between a transparent ITO electrode and highly doped Si electrodes. Silicon has indirect band gap in its bulk state, which prevents it from emitting light. However, as the size of Si particles is reduced to 2-7 nm, the Si quantum dots embedded in the dielectric can emit light with optical or electrical excitations. The EL spectrum is shown in FIG. 14B shows an EL spectrum that is slightly broader than the corresponding PL spectrum from the same materials. The peak emission can be tuned by changing the Si nanoparticles sizes using various processes.
FIG. 15 is a partial cross-sectional view of a finite difference time domain (FDTD) numerical model using the three-layer geometry shown in FIG. 14A. A SiOx layer with Si nanoparticles lies directly on top of a Si substrate, and is covered by ITO for electrical excitation. The models use point sources inside the SiOx layer to represent emission from the Si nanoparticles. As an example, at the operation wavelength of 750 nm, Si has a complex relative permittivity of ∈Si=14.4−i0.09 and ITO shows a relative permittivity of ∈ITO=4.0−i0.017. However, since the light coupled to the substrate is considered lost and the ITO layer is very thin (typical ˜50 nm), the losses due to these materials are negligible in the simulations.
For simulation purposes, the thicknesses of the SiOx layers in these examples are assumed to be 80 nm. The typical radiated fields are shown. Extraction efficiency to air at the operation wavelength of 750 nm is calculated to be around 19.7% for SiOx on Si, and the rest of power is lost in the Si substrate.
Photodetector collection efficiencies for the device of FIG. 14A have been calculated using photodetectors and geometries depicted in FIG. 5. A 10 mm-diameter detector is located at a distance of 20 mm above the EL device having a size of 0.4 mm×0.4 mm. The calculations show that ˜3.2% of the power emitted into the air can be collected by the detector. The overall collected power in the detector to the radiated power from a source embedded in the SRO cases is around 0.7%.
Integrated planar optical circuits also attract interest as a compact on-chip optical interconnector or microfluidics bio/chemical sensors, to name a few examples. SiOx LEDs with embedded nano-scaled Si particles as emission centers provide a very valuable light source for fully CMOS compatible on-chip integrations. However, it is a challenge to efficiently couple light into planar waveguides for optical processing. In typical SiOx LEDs, the active SiOx layers are sandwiched between a top electrode (normally metal, preferably ITO for low loss) and a bottom highly doped Si electrodes. No waveguiding mechanisms for the emitted light exist due to the incompatible optical index contrast between Si and SiOx.
It would be advantageous if a SiOx device could efficiently emit light into air to a photodetector or couple light into a waveguide.
It would be advantageous if a planar waveguide could be fabricated that was compatible with conventional CMOS IC devices.